In the production of oil and gas, chemicals such as corrosion inhibitors, scale inhibitors, paraffin inhibitors, hydrate inhibitors, and demulsifiers are typically injected into the wells to maintain efficient flow of oil or gas. These chemicals usually need to be added to the wells production at a constant rate. Often one pump is used to inject the same chemical into several wells with the use of pressure compensated rate control valves at each injection point. The use of these rate controllers reduces set up and operating costs of injection systems because the alternative is to install a separate pump for each injection point and to maintain several pumps instead of one. These injection valves must be pressure compensated because they need to maintain a rate set point with changes of several thousand pounds per square inch across them to accommodate fluctuations in well pressure. A typical chemical injection rate for an oil well is between 0.5 to 200 US gallons per day. Injection pressures range between 500 to 20,000 psi.
The most robust method to date to achieve rates in this range using pressure compensated rate controllers is to govern the pressure drop across a fixed orifice. The set point for this method is changed by varying the pressure drop across the orifice. This method is described in U.S. Pat. No. 4,893,649. Previous methods to vary the area while maintaining a constant pressure drop have not adequately worked in the low flow range because passages created by mating needles and trims or mating threads to restrict flow are often less than 0.001 inches wide, which makes them prone to clogging and/or filming. The fixed orifice method is robust since hole passage can be made to pass the largest debris for a given flow area and several holes cascading in series can be used to give the same resistance with as much as a twenty fold increase in the flow area reducing the filming and clogging tendencies. The consequences of varying the pressure drop across a fixed resistor is that the range of flow rate set point is limited and passages cannot be opened up to pass blockages as can be done with a mating needle and trim.
Set point range of a valve is defined by its “turn down,” which equals the valve's highest flow rate divided by the lowest flow rate achievable. For a fixed valve orifice, the turn down is calculated by taking the square root of the highest pressure drop across the orifice divided by the lowest pressure drop. For example, a valve that offers a pressure drop across the orifice of 200 psi at maximum flow and 2 psi at minimum flow will have a turn down of 10:1. During the life of the well the flow rate range may need to be adjusted, which involves replacing an orifice. Sending personnel or equipment to remote locations to change an orifice represents a substantial expense, particularly if the valve location is under water.